The present invention relates generally to CDMA cellular telecommunications systems in which multiple microcells (xcexc-cells) can be located within an existing macrocell (M-cell), and operate in the same frequency band, and, more particularly, to such systems in which the microcell size and location is selected, and the uplink and downlink transmission power is controlled, so that the respective microcell and macrocell users can communicate without noticeable interference between each other.
Cellular digital communications systems using CDMA in accordance with the IS-95 standard are currently being deployed for use in the 900 MHz cellular band and 2 GHz PCS band. As the popularity of this type of cellular communication increases, and the number of users and the traffic load increases, there is a need to expand the ability of an existing CDMA cell to carry even more traffic. One proposed solution is to overlay a CDMA M-cell with a TDMA xcexc-cell and vice versa, as described by C.-L. I, L. J. Greenstein, and R. D. Gitlin in xe2x80x9cA microcell/macrocell cellular architecture for low- and high-mobility wireless users,xe2x80x9d IEEE Selected Areas in Communications, vol. 11, no. 6, pp. 885-891, August 1993. However, there has not been a technique that allows a plurality IS-95 microcells to be embedded within an existing IS-95 macrocell which uses the same CDMA band. Such a hierarchical architecture would be useful in a number of applications, such as installing local wireless PBXs and handling localized areas of high usage within a macrocell.
In accordance with the present invention, multiple microcell base stations are located within a macrocell having a single macrocell base station in an hierarchical architecture, and microcell users (xcexc-users) and macrocell users (M-users) communicate respectively with the abase and the M-base in the same frequency band, by appropriately (a) selecting the ratio of the radius r of each xcexc-cell and the distance d between the M-base and the xcexc-base (r and d are measured by the xe2x80x9cradio distancexe2x80x9d, which includes the distance dependent path loss and effects of shadowing), and (b) controlling the power level with which uplink (mobile to base) and downlink (base to mobile) messages are communicated so that the combined capacity of the M-cell and xcexc-cell(s) are high.
In accordance with one aspect of the present invention, xcexc-cell size and location are chosen such that the distance d between M-base and the xcexc-base is a multiple of the radius r of the xcexc-cell. As a minimum, the system is arranged so that d/r greater than 5; ideally, the locations of the M-base and the xcexc-base are chosen so that d/r greater than 10. For a typical application, the microcell is intended to located within a building, so that r is typically between 50 and 100 meters. By way of comparison, a macrocell typically has a radius of between 100 and 3000 meters.
In accordance with another aspect of the invention, with respect to uplink communications, the transmit powers of the xcexc-users in a xcexc-cell are controlled so that the total received power at the nearest M-base is some fixed power level equivalent to the received power from C M-users. Hence, the M-cell basically loses C users worth of capacity, but the total system capacity is increased by virtue of the additional xcexc-users.
In accordance with yet another aspect of the invention, if the microbase and the macrobase are coordinated systems such that they can communicate with each other via a mobile telephone switching office (MTSO) with appropriate signaling messages, a M-user would be handed off to the xcexc-base if the M-user begins to cause excessive interference to the xcexc-base. This arrangement avoids the pathological case of a single mobile terminal causing enough interference to bring down an entire system.
In accordance with yet another aspect of the invention, where the M-base and xcexc-base have no knowledge of each other and handoffs are not allowed between the two base types, a family of I/Q short codes which are quasi-orthogonal to the existing M-base short (pilot) codes are generated for the xcexc-base, and these codes are used to distinguish the xcexc-base from the M-base.
Analysis has shown that with the arrangement of the present invention, an easy to implement and scalable system for embedding multiple CDMA microcells within a macrocell can operate such that macrocell and microcell users communicate within the same frequency band and yet interference is at an acceptable level. Accounting for shadow fading, and for a 1% outage probability, the total capacity of a macrocell with five embedded microcells arranged in accordance with the present invention is more than double the capacity of the macrocell alone.